The present invention is in the field of glass-plastic composites and particularly relates to the manufacture of curved plastic elements of optical quality for laminated glass-plastic lenses.
Glass-plastic lenses have been the subject of research because they combine the chemical durability of glass with the toughness, light weight and tintability of plastics. An early example of such a composite is the laminated glass-plastic lens described by Rogers in U.S. Pat. No. 2,263,249, which combines a glass lens element with a curved plastic element composed of a light-polarizing polyvinyl alcohol plastic sheet, providing a durable polarizing lens. More recent examples of laminated glass-plastic lenses include the glass-plastic safety lens described by French et al. in U.S. Pat. No. 3,867,235, consisting of a conventional glass lens backed by a clear, tough polymeric film.
A number of tough, optically clear thermoplastic films have been considered for composite fabrication, including, for example, cellulosic, polycarbonate, acrylic and ionomer films. The use of such films in the fabrication of curved composite lenses for ophthalmic applications involves a substantial problem of optical quality. The direct lamination of a composite lens using flat thermoplastic film stock or ordinary commercial quality does not typically provide a distortion-free product suitable for ophthalmic use.
Supplemental calendering with rollers can produce flat thermoplastic film of good optical quality. However, with the exception of cast acrylic films, ordinary commercial thermoplastic film manufactured by conventional blowing, casting, or extrusion methods typically exhibits surface defects such as streaks, dimples and the like, rendering the film unsuitable for ophthalmic use. Such defects cannot be removed from tough thermoplastic film by ordinary hot-pressing between polished flat plates. At hot-pressing temperatures high enough to thermally remove manufacturing defects, irregular loop-type markings are produced in the film, perhaps due to the presence of trapped air, which remain in the film over the entire reforming temperature range up to temperatures at which plastic foaming occurs.
In addition to excellent film quality, good optical performance in a laminated lens requries close curvature matching between glass and plastic elements. This matching is aided if preformed curved plastic elements are used in lamination. Vacuum reforming processes have been used to reform relatively thick flat film (e.g. 30 mil cellulose acetate butyrate) into spherically curved plastic elements for sunglass lenses and the like, but such processing is not adaptable to the reforming of thin thermoplastic films, particularly within the thickness range of about 1-15 mils. Moreover, the curvature of the reformed piece may not be precisely controlled, and the reformed plastic has not been of ophthalmic quality. Thin films tend to wrinkle and then stick to reformer surfaces in conventional vacuum reforming equipment. Release coatings which might alleviate this sticking problem, such as fluorocarbon coatings, impart objectionable surface texture to the soft plastic during forming.